1. Field of the Invention
The present invention relates to thin film magnetic heads. Specifically, the present invention relates to the structure of a thin film magnetic head that detects a magnetic field by the change in the relative angle of the magnetization direction in two free layers.
2. Description of the Related Art
As the high recording density of a hard disk drive (HDD) has been developed, a gain of performance of thin film magnetic heads has been required. A composite type thin film magnetic head that has the laminated structure of two heads, a reproducing head that has a magnetoresistive effect element (MR element) for reading and a recording head that has the induction type electromagnetic conversion element for writing, is widely used.
It is known in recent years that an upper shield film, a MR element, and a lower shield film are electrically connected in series, and a head structure in which an insulation layer between the shield films is unnecessary. Such a structure is called a current perpendicular to plane (CPP) structure. To achieve the recording density of 700 Gbits/in2 or more, a giant magnetoresistive magnetoresistive (CPP-GMR) element that uses the CPP structure is an indispensable technology.
The laminated structure of a typical CPP-GMR element is as followings: a lower electrode layer/a base layer/an antiferromagnetic layer/a pinned layer/a spacer layer/a free layer/a cap layer/an upper electrode layer. In the specification, the mark of A/B/C means that the each level of layer, A, B, and C is laminated in this order. As to the pinned layer, the magnetization direction is fixed by exchange coupling with the antiferromagnetic layer. As to the free layer, the magnetization direction is fixed to a direction that is roughly orthogonal in the magnetization direction of the pinned layer if the external magnetic field is not applied. However, once the external magnetic field is applied, it rotates the magnetization direction according to the external magnetic field. This layer structure is called a spin valve (SV) or SV layer.
In an actual head, the distance between the upper electrode layer and the lower electrode layer, which function as the shield, influences BPI (bit per inch: track recording density) directly. This distance is called a read gap, and making a narrow read gap is an essential requirement for HDD with a high recording density. In the CPP-GMR element mentioned above, the antiferromagnetic layer is needed for the fixation of the magnetization direction of the pinned layer, and therefore, it is an obstacle for making the narrow read gap. It is expected that a minimum read gap is about 20 nm or more as long as the SV layer is adopted, and there is a possibility that the read gap demarcates the high recording density limit.
The specification of U.S. Pat. No. 5,576,914, the specification of U.S. Pat. No. 6,724,583, and others disclose the structure, such as a lower electrode layer/a base layer/a first free layer/a spacer layer/a second free layer/a cap layer/an upper electrode layer. In this structure, the magnetization direction in two free layers is changed according to the external magnetic field, and the output is decided depending on the relative angle of the magnetization direction in two free layers. It is possible to make a narrow read gap vastly because the antiferromagnetic layer is not required in this structure. In this specification, the layer structure that has two such free layers is called a dual free structure.
In order to have the situation where the magnetization direction in two free layers is rotated according to the external magnetic field, and the response to the magnetic field from the medium is maximized, it is ideal that the magnetization directions in two free layers are nearly orthogonal to each other when the magnetic field from the medium is zero (0). After the two free layers are antiferromagnetically coupled and the magnetization directions are mutually antiparallel, the bias magnetic field is applied to the two free layers in an orthogonal direction to the opposite side of the medium (or an air bearing surface; hereinafter ABS). The magnetization directions of the two free layers are provided in the mutually orthogonal direction by the above described operations.
The effect of Ruderman-Kittel-Kasuya-Yosida (RKKY) through the spacer layer can be used to antiferromagnetically couple the two free layers. For instance, in the situation where the layer structure is CoFe/Ru/CoFe, it is known that if the layer thickness of Ru is 0.7˜0.9 nm, the magnetization directions in the two CoFe layers are antiparallel due to the RKKY effect. This phenomenon is used to construct the synthetic pinned structure in the SV layer. It is also well-known that the layer structure of CoFe/Cu/CoFe can achieve an antiferromagnetic coupling. Because Cu is a material generally used as a spacer layer, the CoFe/Cu/CoFe structure or its similar layer structure can be used for a free layer/a spacer layer/a free layer of the above mentioned dual free structure.
However, a spacer material in the layer structure of the dual free structure that fulfills many requirements is required severe demands, such as showing the RKKY effect, transmitting spin information of the electron efficiently, and having an ideal resistance as an MR element. Although Cu shows the RKKY effect and Cu is excellent in transmitting spin information, Cu is not practical due to the low output because the resistance is too low. Ru is poor in transmitting spin information and its resistance is too low. Under the current technology, it is difficult to provide the thin film magnetic head that has the output performance equal with a conventional CPP-GMR element and that is easy to make the narrow read gap.